Several types of photochromic compounds are known for use in applications in which reversible color change, or darkening, induced by sunlight is desirable. These applications include for example, ophthalmic lenses, solar protection lenses, filters, camera optical systems, decorative objects, windows and the like.
However, the known compounds are disadvantageous in that they do not exhibit both high optical density and the ability to return to their original color quickly once the activating light source is removed. Thus, there is a need for new compounds with fast response to ultraviolet (“UV”) irradiation, high colorability and long lifetime.